JPWO2011132283A1 - LIGHT MODULATION DEVICE, LIGHT MODULATION DEVICE DRIVE METHOD, AND LIGHT MODULATION DEVICE MANUFACTURING METHOD - Google Patents

Abstract

When adjusting the phase of light passing through the arms (12a, 12b), the arm (12a) modulates the first DC bias and the first modulation signal from the modulation electrode (13a), and the arm (12b) modulates the light. A second DC bias and a second modulation signal are applied from the electrode (13b), respectively. The first and second DC biases are the product of the length of the modulation electrode (13a) and the first DC bias, and the modulation. The modulation electrode (13a, 13b) is applied so that the ratio of the product of the length of the electrode (13b) and the product of the second DC bias is kept constant. This configuration realizes optimal control of the phase difference between the arms (12a, 12b) and precise control of the wavelength chirp characteristics with a simple element structure in an optical modulator using a semiconductor Mach-Zehnder modulator. Thus, optical modulation with a small device size and good characteristics is realized.

Description

  The present invention relates to an optical modulation device including a Mach-Zehnder type optical waveguide, a driving method thereof, and a manufacturing method thereof.

A so-called Mach-Zehnder type modulator was used as an optical modulator operating at high speed in a wide wavelength range, which is strongly demanded as a communication light source for a WDM system capable of large-capacity optical communication. Optical modulators are attracting attention. At present, in a long-distance and large-capacity optical communication system, for example, a Mach-Zehnder interferometer type optical modulator (hereinafter simply referred to as an LN−) utilizing an electro-optic effect in a nonlinear optical crystal lithium niobate (LiNbO ₃ , LN). An optical transmitter including an MZ type optical modulator) and a semiconductor laser is used. Further, in recent years, development of a semiconductor Mach-Zehnder (MZ) type optical modulator (hereinafter simply referred to as a semiconductor MZ type optical modulator) and a semiconductor laser integrated with a semiconductor MZ type optical modulator that can be reduced in size has progressed. It has been.

An example of the semiconductor MZ type optical modulator is shown in FIG.
This semiconductor MZ type modulator includes a first optical coupler 101, arms 102a and 102b, a second optical coupler 104, and modulation electrodes 103a and 103b formed of a semiconductor waveguide.
The first optical coupler 101 has an input port 101a, and is an input coupler that branches incident light into two. The arms 102a and 102b are waveguides through which two branched lights propagate. The modulation electrodes 103a and 103b are formed on the waveguides of the arms 102a and 102b, and are electrodes for applying modulation signals to the arms 102a and 102b, respectively. The second optical coupler 104 has an output port 104a, and is an output coupler that combines (combines) light propagated through the arms 102a and 102b.

  One end of each of the two arms 102 a and 102 b is connected to the first optical coupler 101 and the other end is connected to the second optical coupler 104. The light that has entered the first optical coupler 101 from the input port 101a is branched by the arms 102a and 102b, passes through the arms 102a and 102b, and is then combined by the second optical coupler 104 and output from the output port 104a. To do. The output light is switched on / off according to the interference state of light at the time of multiplexing. If the phase difference when the light that has passed through the arms 102a and 102b is recombined at the output port 104a of the second optical coupler 104 is 0 (or 2Nπ, N is an integer), the interference state is intensifying each other. The light is output from the output port 104a. On the other hand, if the phase difference when the light that has passed through the arms 102a and 102b is recombined at the output port 104a of the second optical coupler 104 is π (or (2N + 1) π), the interference state is weakened. For this reason, the light is not output from the output port 104a.

  As a method for adjusting the phase of light passing through the arms 102a and 102b, for example, there is a method of adjusting the phase by using a change in refractive index generated when an electric field is applied to the arms 102a and 102b. Therefore, if a modulation voltage signal is applied to the arms 102a and 102b by the modulation electrodes 103a and 103b, a phase change occurs in the arms 102a and 102b according to the modulation voltage signal, and the intensity of the light output as a result is modulated. . In order to perform good optical modulation, first, the modulation signal applied to the arms 102a and 102b needs to have a sufficiently large amplitude to change the phase difference by π. In addition, it is necessary to control the phase difference of the light that has passed through the arms 102a and 102b so that the phase difference becomes π at the off level of the modulation voltage signal and becomes zero at the on level. Since the phase difference of the light passing through the arms 102a and 102b differs for each modulator element depending on the manufacturing error or the like, it is necessary to adjust the phase for each modulator element.

  As a method for controlling the phase difference between the arms 102a and 102b, a phase control electrode is formed on at least one arm separately from the modulation electrodes 103a and 103b, and the voltage applied to the phase control electrode is adjusted. There is a method of changing the phase by changing the refractive index of at least one of the arms. As another method for controlling the phase difference between the arms 102a and 102b, there is a method of making a difference in the DC bias applied to the modulation electrodes 103a and 103b. In general, a semiconductor MZ type modulator performs a modulation operation by applying a DC bias of about several volts. A phase difference between the arms 102a and 102b is obtained by adding a difference between the DC biases between the arms 102a and 102b. Can be adjusted.

  On the other hand, in the MZ type optical modulator, a so-called wavelength chirp phenomenon occurs in which the wavelength of the output light fluctuates during modulation. This wavelength chirp becomes a factor that degrades the optical modulation waveform after the optical fiber transmission. Therefore, in order to obtain sufficient modulation characteristics in the MZ type optical modulator, it is necessary to precisely control the wavelength chirp. As the technology for controlling the wavelength chirp, there are a zero chirp operation technology for minimizing the amount of wavelength chirp and a negative chirp operation technology for intentionally adding a wavelength chirp so that the optical waveform after transmission is improved. These chirp control techniques are realized by adjusting the amplitude ratio of the phase change of the light generated in the arms 102a and 102b when a modulation voltage signal is applied. In the zero chirp operation, it is desirable to fix the amplitude ratio of the phase change of the light generated in the arms 102a and 102b to 1: 1 when a modulation signal that is a high frequency voltage is applied. In the negative chirp operation, it is desirable to fix the ratio of the amplitude of the phase change of the light generated in one arm and the other arm, for example, about 0.85: 0.15.

The above ratio is realized, for example, by adjusting the ratio of the amplitude of the voltage signal applied to the arms 102a and 102b. In order to realize the zero chirp operation, a modulation signal having the same amplitude and the opposite direction is applied to the arms 102a and 102b, and the amount of phase change generated in the arms 102a and 102b is the same and the direction is reversed. To be. In order to realize the negative chirp operation, for example, a modulation signal having different amplitudes and opposite directions is applied between the arms 102a and 102b, and the phase change amount of one of the arms 102a and 102b increases. Like that.
As described above, in the MZ type optical modulation device, in order to obtain good modulation characteristics, both the control of the phase difference between the arms 102a and 102b and the control of the ratio of the amplitude of the phase change between the arms are appropriately performed. Is required.

Special table 2007-531022 gazette

In recent years, there has been a strong demand for downsizing of an optical transceiver module equipped with the semiconductor MZ modulator as described above.
In order to reduce the size of the optical transceiver module as described above, it is first necessary that the modulator itself be small. In addition to downsizing the modulator, it is important to downsize and simplify the modulator module including the modulator. In this case, among the phase control methods described above, the method using the phase control electrode separately from the modulation electrode increases the number of electrode pins for connection in the modulator module, and is not suitable for miniaturization. is there. Further, since the control parameter of the modulator is increased by one with the addition of the phase control electrode, there is a problem that the configuration of the control circuit becomes complicated.

Consider a method of adjusting the phase in which a DC bias is applied to the modulation electrode. In this case, it is suitable for downsizing the package of the semiconductor MZ type optical modulator, but there is a problem peculiar to the semiconductor MZ type modulator that it is difficult to control the wavelength chirp as described below.
In a semiconductor MZ type optical modulator, a modulation operation is generally realized by applying a modulation signal having a reverse voltage to an optical waveguide to change the refractive index. Here, the application of the reverse voltage modulation signal means that a negative bias is applied to the p-side electrode and a positive bias is applied to the n-side electrode of the semiconductor MZ type optical modulator. Hereinafter, applying a reverse voltage to the optical waveguide is simply referred to as applying a voltage.

FIG. 20A shows how the phase changes when a modulation signal is applied to the optical waveguide of the modulator. As shown in the figure, in the optical waveguide, a phase change occurs in a relation of a quadratic function with respect to the modulation signal. In other words, the efficiency of phase change increases in proportion to the applied DC bias. That is, even when modulated voltage signals having the same amplitude are applied to the optical waveguide, the amount of phase change that occurs depends on the applied DC bias.
In the modulator, when the phase difference is adjusted by the DC bias, the DC bias applied to the modulation electrodes of the two arms as the optical waveguide is shifted and adjusted. However, when the DC bias is shifted, the reason is as described above. The phase change efficiency relationship changes between the two arms.

In the case of the modulator in which the lengths of the modulation electrodes 103a and 103b are the same as in FIG. 19, the relationship between the voltage applied to the arms 102a and 102b and the phase change is the same. A necessary amount of adjustment of the phase difference, that is, a necessary difference in DC bias differs depending on each semiconductor MZ optical modulator due to a manufacturing error or the like. For example, as shown in FIG. 20A, it is assumed that a necessary amount of phase difference adjustment is Δφ in a certain modulator. At this time, when the DC bias applied to the arms 102a and 102b is V _DC1 and V _DC2 , the phase difference becomes an appropriate value. On the other hand, in another modulator, the necessary adjustment amount of the phase difference is Δφ ′, and the phase difference becomes an appropriate value when V _DC1 and V _DC2 ′.
As a result, as shown in FIG. 20B, the phase change efficiency relationship between the arms 102a and 102b differs for each modulator. In FIG. 20B, the efficiency of phase change is the rate of change (differential value) of the phase in FIG. 20A with respect to the DC bias.

  As shown in the background art, in order to control the wavelength chirp, it is necessary to precisely adjust the ratio of the phase change amount generated in the two arms (waveguides). When the phase control is performed with the DC bias as described above, the phase change efficiency with respect to the voltage signal between the two arms is shifted, and the amplitude ratio of the phase change between the arms changes for each MZ optical modulator. End up. For this reason, the ratio of the phase change amounts generated by the two arms cannot be adjusted precisely, and good chirp characteristics cannot be obtained.

Regarding the change in the efficiency of the phase change, it is conceivable to adjust the phase change amount by changing the ratio of the magnitude of the modulation signal. However, in this case, a complicated driving circuit that generates two types of modulation voltage signals and can arbitrarily adjust the amplitudes of the two types of modulation voltage signals is necessary. However, such a circuit is difficult to realize. is there. In particular, it is almost impossible to realize a drive circuit that can be mounted on a small optical transceiver. Thus, it is difficult to precisely control the wavelength chirp in the phase control by the DC bias.
Therefore, at present, in a semiconductor MZ optical modulator, it is difficult to appropriately adjust both phase control and wavelength chirp without using a complicated control circuit with a small and simple element structure.

  The present invention has been made in view of the above problems, and in a semiconductor MZ type modulator, optimal control of the phase difference between the first and second optical waveguides and precise control of the wavelength chirp characteristics are provided. To provide a light modulation device, a light modulation device control method, and a light modulation device manufacturing method capable of realizing light modulation having a small device size and good characteristics, which can be realized with a simple element structure. With the goal.

  One aspect of the light modulation device is a light modulation device using a semiconductor Mach-Zehnder type modulator that branches and propagates input light, and combines and outputs the propagated light, the semiconductor Mach-Zehnder The modulator includes a first optical waveguide and a second optical waveguide that propagate the branched light, a first electrode that applies a first modulation signal and a first DC bias to the first optical waveguide, A second electrode for applying a second modulation signal and a second DC bias to the second optical waveguide, and the product of the length of the first electrode and the first DC bias; The first DC bias and the second DC bias are applied so that the ratio of the product of the length of the two electrodes and the product of the second DC bias is kept constant.

  One aspect of a method for driving an optical modulator is a method for driving an optical modulator using a semiconductor Mach-Zehnder modulator that branches and propagates input light and combines and outputs the propagated light. A first optical waveguide and a second optical waveguide for propagating the branched light, a first electrode for applying a first modulation signal and a first DC bias to the first optical waveguide, and the second An optical modulation device including a second electrode for applying a second modulation signal and a second DC bias to the optical waveguide of the optical waveguide, the product of the length of the first electrode and the first DC bias; The first DC bias and the second optical waveguide are kept in the first optical waveguide and the second optical waveguide so that a ratio of a product of the length of the second electrode and the product of the second DC bias is kept constant. 2 DC bias is applied.

  One aspect of a method for manufacturing a light modulation device is a method for manufacturing a light modulation device using a semiconductor Mach-Zehnder modulator that divides and propagates input light and combines and outputs the propagated light. Forming the first optical waveguide and the second optical waveguide for propagating the branched light when manufacturing the semiconductor Mach-Zehnder modulator, and forming the first optical waveguide on the first optical waveguide. A first electrode for applying a first modulation signal and a first DC bias to the optical waveguide; and a second modulation signal and a second signal on the second optical waveguide on the second optical waveguide. Forming a second electrode for applying a DC bias, and further comprising a product of the length of the first electrode and the first DC bias, the length of the second electrode, and the A ratio of the product of the second DC bias and the first DC buffer is maintained at a constant value. Including astigmatism and the step of determining the second DC bias.

  According to each of the above aspects, it is possible to realize optimal control of the phase difference between the first and second optical waveguides and precise control of the wavelength chirp characteristics with a simple element configuration, and the size of the device. Can achieve light modulation with small size and good characteristics.

FIG. 1 is a schematic plan view showing a semiconductor MZ modulator used in the light modulation device according to the first embodiment. FIG. 2A is a characteristic diagram showing a phase with respect to an applied voltage when V _DC1 : V _DC2 = 1: 1. FIG. 2B is a characteristic diagram showing the efficiency of phase change with respect to the applied voltage when V _DC1 : V _DC2 = 1: 1. FIG. 3A is a characteristic diagram showing a phase with respect to an applied voltage when V _DC1 : V _DC2 = L ₂ : L ₁ . FIG. 3B is a characteristic diagram showing the efficiency of phase change with respect to the applied voltage when V _DC1 : V _DC2 = L ₂ : L ₁ . FIG. 4A is a schematic plan view showing the semiconductor MZ modulator used in the light modulation device according to the first embodiment. FIG. 4B is a schematic cross-sectional view showing the semiconductor MZ modulator used in the light modulation device according to the first embodiment. FIG. 5 is a schematic plan view showing a modulator module on which the semiconductor MZ modulator of FIGS. 4A and 4B is mounted. FIG. 6 is a schematic plan view showing an optical modulation device on which the modulator module of FIG. 5 is mounted. FIG. 7 is a schematic plan view illustrating another example of the light modulation device according to the first embodiment. FIG. 8 is a flowchart showing the method of manufacturing the light modulation device according to the first embodiment in the order of steps. FIG. 9 is a characteristic diagram showing the phase and the efficiency of phase change when V _DC1 : V _DC2 = A: B = 1: 1 in the first embodiment. FIG. 10 is a characteristic diagram showing an extinction curve used when adjusting the phase difference in the first embodiment. FIG. 11 is a flowchart showing a driving method of the MZ type optical modulation device according to the first embodiment. FIG. 12 is a characteristic diagram showing an extinction curve used when adjusting the phase difference in the second embodiment. FIG. 13 is a characteristic diagram showing the phase and the efficiency of phase change when V _DC1 : V _DC2 = 2.5: 1 in the third embodiment. FIG. 14 is a characteristic diagram showing an extinction curve used when adjusting the phase difference in the third embodiment. FIG. 15 is a schematic plan view showing a semiconductor Mach-Zehnder modulator used in the light modulation device according to the fourth embodiment. FIG. 16 is a schematic plan view showing a modulator module on which the semiconductor Mach-Zehnder modulator of FIG. 15 is mounted. FIG. 17 is a schematic plan view showing an optical modulation device on which the modulator module of FIG. 16 is mounted. FIG. 18 is a schematic plan view showing another example of the light modulation device according to the fourth embodiment. FIG. 19 is a schematic plan view showing an example of a semiconductor Mach-Zehnder modulator used in a conventional light modulation device. FIG. 20A is a characteristic diagram showing a phase relationship with respect to a voltage applied to the optical waveguide of the modulator element. FIG. 20B is a characteristic diagram showing the relationship between the efficiency of the phase change with respect to the voltage applied to the optical waveguide of the modulator element.

-Basic configuration of semiconductor Mach-Zehnder modulator-
FIG. 1 is a schematic plan view showing a semiconductor Mach-Zehnder modulator used in the light modulation device according to the present embodiment.
The semiconductor MZ type modulator 10 includes a first optical coupler 1, arms 2a and 2b, a semiconductor optical waveguide constituting the second optical coupler 4, and modulation electrodes 3a and 3b.
The first optical coupler 1 has an input port 1a and is an input coupler that branches incident light into two. The arms 2a and 2b are waveguides through which two branched lights propagate. The modulation electrodes 3a and 3b are electrodes for applying modulation signals to the arms 2a and 2b, respectively. The modulation electrode 3a and the modulation electrode 3b are formed to have different lengths as will be described later. The second optical coupler 4 has an output port 4a, and is an output coupler that combines (combines) light propagated through the arms 2a and 2b.

  One end of each of the two arms 2 a and 2 b is connected to the first optical coupler 1 and the other end is connected to the second optical coupler 4. The light incident on the first optical coupler 1 from the input port 1a is branched by the arms 2a and 2b, passes through the arms 2a and 2b, respectively, and is multiplexed by the second optical coupler 4 and output from the output port 4a. To do. The output light is switched on / off according to the interference state of light at the time of multiplexing. If the phase difference when the light that has passed through the arms 2a and 2b is combined at the output port 4a is 0 (or 2Nπ, N is an integer), the interference state is intensifying each other. Is turned on. On the other hand, when the phase difference when the light that has passed through the arms 2a and 2b is combined at the output port 4a is π (or (2N + 1) π), the interference state is mutually debilitating. The light enters an off state where no light is output. Thus, the light incident from the input port 1a is output from the output port 4a with the intensity changed according to the phase difference between the arms 2a and 2b.

In order to adjust the phase of the light passing through the arms 2a and 2b, the arm 2a has the first DC bias and the first modulation signal from the modulation electrode 3a, and the arm 2b has the second DC bias from the modulation electrode 3b. And the second modulation signal are applied.
In the arms 2a and 2b, the ratio of the product of the length of the modulation electrode 3a and the first DC bias and the product of the length of the modulation electrode 3b and the second DC bias is kept constant. A first DC bias and a second DC bias are applied. Here, the DC bias refers to a DC bias value at the midpoint of the on / off of the modulation signal, that is, a center bias.

Since the phase change of the light in the semiconductor optical waveguide is substantially proportional to the square of the applied voltage, the efficiency of the phase change with respect to the voltage change increases in proportion to the DC bias _VDC . Further, the efficiency of phase change increases in proportion to the length L of the modulation electrode. That is, the phase change efficiency η is proportional to the product of the DC bias V _DC and the length L of the modulation electrode as follows.
η = αV _DC L α: Proportional coefficient (1)

If the lengths of the modulation electrodes 3a and 3b of the arms 2a and 2b are L ₁ and L ₂ and the DC bias applied to the modulation electrodes 3a and 3b is V _DC1 and V _DC2 , the efficiency of phase change of the arms 2a and 2b η ₁ and η ₂ are as follows. Here, the “length of the modulation electrode” refers to the length along the longitudinal direction of the arm where the modulation electrode is electrically connected to the waveguide forming the arm. The same applies to the following embodiments.
η ₁ = αL ₁ V _DC1 (2)
η ₂ = αL ₂ V _DC2 (3)

Here, when V _DC1 and V _DC2 are fixed at a constant ratio, V _DC1 : V _DC2 = A: B, the ratio of η ₁ and η ₂ is as follows.
η ₁ : η ₂ = AL ₁ : BL ₂ (4)
Since the lengths of the modulation electrodes 3a and 3b are determined when the modulator element 10 is manufactured, L ₁ and L ₂ are fixed values. Therefore, when the ratio of the DC bias between the two arms is fixed, the ratio of η ₁ and η ₂ , that is, the product of the length L ₁ and the first DC bias V _DC1 , and the length L ₂ And the product of the second DC bias V _DC2 is fixed to a constant value.

As described above, in a state of fixing the first and second DC ratio bias _{V DC1, V DC2,} arm 2a, when increasing or decreasing the _{V DC1, V DC2} applied to ^{_{2b, A: B = L 2}} 1 ^{/ 2} : The phase difference between the arms 2a and 2b can be changed except when L ₁ ^½ .

As a specific example, the efficiency of phase and phase change in the case of V _DC1 : V _DC2 = A: B = 1: 1 is shown in FIG. 2A and FIG. 2B. V _DC1 : V _DC2 = A: B = L ₂ : L ₁ The phase and the efficiency of phase change in the case are shown in FIGS. 3A and 3B, respectively. 2A and 3A show the phase value with respect to the center bias, and FIGS. 2B and 3B show the efficiency of the phase change with respect to the center bias. The efficiency of phase change is the rate of change (differential value) of the phase in FIG. 2A and FIG. 3A with respect to the DC bias.

As shown in FIGS. 2A and 3A, when the ratio of the DC biases V _DC1 and V _DC2 applied to the arms 2a and 2b is fixed, the arm 2a is increased by increasing V _DC1 and V _DC2 while maintaining the relationship. , 2b can be increased.
That is, in FIG. _{2A, V DC1: V DC2 =} 1: If the DC bias holds one relationship was increased from _V DC1 _{(= V DC2)} to _{V DC1 '(= V DC2)} , the arms 2a, 2b The phase difference between them increases from Δφ to Δφ ′.
In FIG. 3A, when the relationship of V _DC1 : V _DC2 = L ₂ : L ₁ is maintained and the DC bias V _DC1 is increased to V _DC1 ′ and the DC bias V _DC2 is increased to V _DC2 ′, between the arms 2 a and 2 b The phase difference increases from Δφ to Δφ ′.

Consider the efficiency of phase change.
Consider the case where V _DC1 : V _DC2 = 1: 1 in the modulator element 10. At this time, the ratio η ₁ : η ₂ of the efficiency of the phase change generated in the arms 2a and 2b is fixed at the ratio L ₁ : L ₂ of the lengths of the modulation electrodes 3a and 3b from the equation (4). That is, as shown in FIG. _{2B, V DC1: V DC2 =} 1: the DC bias holds 1 relationship even when increased from _V DC1 _{(= V DC2)} to _{V DC1 '(= V DC2)} , The ratio of the efficiency of phase change is invariant with η ₁ : η ₂ = η ₁ ′: η ₂ ′ = L ₁ : L ₂ .
Therefore, first, the modulation electrodes 3a and 3b are formed such that the lengths L ₁ and L ₂ have a ratio of the amount of phase change (phase change efficiency) suitable for obtaining the optimum chirp characteristics. Under this condition, an appropriate chirp characteristic can be obtained by applying the first modulation signal and the second modulation signal having the same amplitude to the arms 2a and 2b by the modulation electrodes 3a and 3b. In particular, if the modulation electrodes 3a and 3b are formed so that the ratio L ₁ : L ₂ is about 1: 4 to 1: 5, the chirp characteristics equivalent to the chirp characteristics of the negative chirp operation in the LN-MZ type light modulation device. (Α parameter = −0.7) can be obtained.

In the modulator element 10, when the ratio of the DC bias applied to the modulation electrodes 3a and 3b is opposite to the ratio of the lengths of the modulation electrodes 3a and 3b, that is, V _DC1 : V _DC2 = L ₂ : L ₁ think about. At this time, the ratio η ₁ : η ₂ of the efficiency of phase change between the arms 2a and 2b can be 1: 1. That is, as shown in FIG. 3B, even when the relationship of V _DC1 : V _DC2 = L ₂ : L ₁ is maintained and the DC bias V _DC1 is increased to V _DC1 ′ and the DC bias V _DC2 is increased to V _DC2 ′. The ratio of the efficiency of the phase change is invariable from equation (4) as η ₁ : η ₂ = η ₁ ′: η ₂ ′ = 1: 1.
Under this condition, the first and second modulation signals having the same amplitude are applied to the arms 2a and 2b by the modulation electrodes 3a and 3b having the same amplitude to the modulation electrodes 3a and 3b. As a result, it is possible to realize a zero chirp operation having a chirp characteristic (α parameter≈0) in which almost no wavelength chirp is generated.
Since the amplitudes of the first and second modulation signals applied to the modulation electrodes 3a and 3b can be set to the same value to obtain optimum chirp characteristics, a simple drive circuit can be used.

  As described above, in the optical modulation device using the semiconductor Mach-Zehnder modulator according to the present embodiment, the phase change between the arms 2a and 2b is fixed with the ratio of the efficiency of phase change between the arms 2a and 2b fixed. It is possible to adjust the phase difference. As a result, the adjustment of the phase difference between the arms 2a and 2b and the precise control of the chirp characteristics can be performed at the same time. Further, when driving the light modulation device, it is only necessary to adjust the DC bias applied to the modulation electrodes 3a and 3b when adjusting the phase difference. For this reason, it is not necessary to separately provide an electrode for phase control, and the modulator and the modulator module can be downsized.

-Specific embodiments-
Hereinafter, specific embodiments will be described in detail with reference to the drawings.

(First embodiment)
4A and 4B are schematic diagrams illustrating the semiconductor Mach-Zehnder modulator used in the light modulation device according to the first embodiment, in which FIG. 4A is a plan view and FIG. 4B is along the broken line II in FIG. 4A. It is sectional drawing.
FIG. 5 is a schematic plan view showing a modulator module on which the semiconductor Mach-Zehnder modulator of FIG. 4 is mounted.
FIG. 6 is a schematic plan view showing an optical modulation device on which the modulator module of FIG. 5 is mounted.

  As shown in FIG. 4A, the semiconductor Mach-Zehnder modulator 20 used in the optical modulation device (optical transmitter) according to the present embodiment includes the first optical coupler 11, the arms 12a and 12b, and the second optical coupler 14. And a modulation electrode 13a, 13b.

  The first optical coupler 11 is a so-called 2 × 2 MMI coupler that has an input port 11a and branches incident light into two. The arms 12a and 12b are optical waveguides through which two branched lights propagate. The modulation electrodes 13a and 13b are formed on the optical waveguides of the arms 12a and 12b, and are electrodes for applying the first modulation signal and the second modulation signal to the arms 12a and 12b. The modulation electrodes 13a and 13b are formed in a so-called traveling wave type microstrip line structure having a portion where a modulation signal is inputted and a portion where the modulation signal is outputted. The modulation electrode 13a and the modulation electrode 13b are formed to have different lengths as will be described later. The second optical coupler 14 has an output port 14a, and is a 2 × 2 MMI coupler that multiplexes (combines) light propagated through the arms 12a and 12b.

  The modulation electrodes 13a and 13b are not limited to the microstrip line structure. For example, a coplanar electrode structure in which a ground electrode is formed on the upper surface in parallel with the modulation electrode, a lumped constant type electrode, or the like may be used.

  One end of each of the two arms 12 a and 12 b is connected to the first optical coupler 11 and the other end is connected to the second optical coupler 14. The light that has entered the first optical coupler 11 from the input port 11a is branched by the arms 12a and 12b, passes through the arms 12a and 12b, and is then combined by the second optical coupler 14 and output from the output port 14a. To do. The output light is switched on / off according to the interference state of light at the time of multiplexing. If the phase difference when the light that has passed through the arms 12a and 12b is multiplexed at the output port 14a is 0 (or 2Nπ, N is an integer), the interference state is intensifying each other. The output is turned on. On the other hand, if the phase difference when the light that has passed through the arms 12a and 2b is combined at the output port 14a is π (or (2N + 1) π), the interference state is mutually debilitating. Is not output. Thus, the light incident from the input port 11a is output from the output port 14a with the intensity changed according to the phase difference between the arms 12a and 12b.

  In the semiconductor Mach-Zehnder modulator 20, as shown in FIGS. 4A and 4B, the first optical coupler 11, the arms 12a and 12b, and the second optical coupler 14 are formed on the surface of the n-InP substrate 21. A waveguide is formed, and an insulating layer 22 is formed so as to cover the side surface of the optical waveguide. Modulation electrodes 13a and 13b are formed on the arms 12a and 12b. A common ground electrode 23 is formed on the back surface of the n-InP substrate 21.

  As shown in FIG. 4B, the arms 12a and 12b are formed as semiconductor optical waveguides having a mesa structure. Here, an n-InP clad layer 12A, an i-InP layer 12B, an InGaAsP-MQW (Multiple-Quantum Well) core layer 12C, an i-InP layer 12D, a p-InP clad layer 12E, The InGaAsP contact layer 12F is sequentially stacked. The InGaAsP-MQW core layer 12C is formed, for example, by laminating a plurality of InGaAsP layers and InP layers.

The insulating layer 22 includes an SiO ₂ film 22a formed from the n-InP substrate 21 to the side surfaces of the arms 12a and 12b, a low dielectric resin film 22b formed on the SiO ₂ film 22a, and a low dielectric resin film. And an SiO ₂ film 22c formed on 22b. The SiO ₂ films 22a and 22b function as passivation films. The low dielectric resin film 22b is formed using, for example, benzocyclobutene (BCB) or the like as a material.

An outline of the manufacturing process of the semiconductor Mach-Zehnder modulator is shown below.
First, semiconductor layers 12A to 12F as shown in FIG. 4B are deposited on the n-InP substrate 21 by using, for example, a general MOCVD method. Thereafter, only a portion where the first optical coupler 11, the arms 12a and 12b, the second optical coupler 14, the input port 11a, and the output port 14a are formed is covered by a general lithography technique using a resist and dry etching. A mask such as SiO ₂ is formed. The layers 12A to 12F are dry-etched using this mask. Thereby, the arms 12a, 12b and the like having a mesa structure are formed.

Subsequently, after removing the mask, the insulating layer 22 is formed on the entire surface by CVD or the like. Then, the insulating layer 22 only on the arms 12a and 12b is removed by a general lithography technique using a resist and dry etching to form an opening.
Thereafter, Ti / Pt / Au deposition and gold plating are performed, for example, to form modulation electrodes 13a and 13b connected to the arms 12a and 12b in the openings.

  In this embodiment, the InGaAsP-based MQW core layer 12C is used as the optical waveguide core layer. However, the present invention is not limited to this, and other semiconductor MQW core layers such as an AlGaInAs-based MQW core layer may be used.

In the semiconductor Mach-Zehnder modulator configured as described above, in order to adjust the phase of the light passing through the arms 12a and 12b, the arm 12a is supplied with the first DC bias and the first modulation signal from the modulation electrode 13a. The second DC bias and the second modulation signal are applied to the arm 12b from the modulation electrode 13b, respectively.
In the arms 12a and 12b, the ratio of the product of the length of the modulation electrode 13a and the first DC bias and the product of the length of the modulation electrode 13b and the second DC bias is maintained at a constant value. A first DC bias and a second DC bias are applied.

  In the present embodiment, a DC bias (center bias) having the same value and a first modulation signal and a second modulation signal having the same amplitude and opposite voltage changes are applied to the modulation electrodes 13a and 13b. Under this condition, the ratio of the efficiency of phase change between the arms 12a and 12b is fixed at the ratio of the lengths of the modulation electrodes 13a and 13b. Accordingly, by appropriately setting the ratio of the modulation electrode length, it is possible to appropriately adjust the ratio of the phase change amount between the two arms and to realize a good negative chirp operation.

The details are as follows. The modulation electrode 13a is formed to have a length L ₁ = 1.5 mm, and the modulation electrode 13b is formed to have a length L ₂ = 0.3 mm. V _DC1 : V _DC2 is fixed at 1: 1. Accordingly, the phase change efficiency ratio η ₁ : η ₂ in the arms 2 a and 2 b is fixed to the following value.
η ₁ : η ₂ = L ₁ V _DC1 : L ₂ V _DC2 = L ₁ : L ₂ = 1.5: 0.3 = 5: 1
As described above (as shown in FIGS. 2A and 2B), by increasing or decreasing the DC bias of both the arms 12a and 12b while keeping the DC bias of the modulation electrodes 13a and 13b matched, the position between the arms 12a and 12b is increased. It can be adjusted by changing the phase difference.

As shown in FIG. 5, an optical modulator module 30 is configured using the semiconductor Mach-Zehnder modulator 20 configured as described above.
The semiconductor Mach-Zehnder modulator 20 is bonded on the carrier 31. The carrier 31 on which the semiconductor Mach-Zehnder modulator 20 is mounted is disposed on a temperature adjusting element, for example, a Peltier element 32. The Peltier element 32 on which the carrier 31 is mounted is mounted in a module package 33, and the optical modulator module 30 is configured.

  On the carrier 31, electrodes 31a, 31b, 31c, 31d are patterned, and a thermistor 38 for monitoring the temperature of the modulator element 20 is provided. For example, the electrode 31a is wire-bonded to one end of the modulation electrode 13a, and the electrode 31b is wire-bonded to the other end of the modulation electrode 13a, for example. For example, a 50Ω termination resistor 39 connected to the carrier 31 is disposed on the electrode 31b. For example, the electrode 31c is wire-bonded to one end of the modulation electrode 13b, and the electrode 31d is wire-bonded to the other end of the modulation electrode 13b, for example. A termination resistor 39 connected to the carrier 31 is disposed on the electrode 31d.

  Collimating lenses 35 are disposed in front of and behind the semiconductor Mach-Zehnder modulator 20, respectively. The module package 33 is provided with optical fibers 34 a and 34 b with lenses for condensing the light that has been collimated by the collimating lens 35. The light output from the optical fiber 34a is coupled to the modulator element 20, and the light output through the semiconductor Mach-Zehnder modulator 20 is coupled to the optical fiber 34b. Alternatively, conversely, the light output from 34b may be coupled to modulator element 20, and the light output after passing through modulator element 20 may be coupled to optical fiber 34a.

  The module package 33 is provided with electrode pins 36a, 36b, 36c, 36d, 37a, 37b, 37c, 37d for driving the semiconductor Mach-Zehnder modulator 20. The electrode pin 36a is wire bonded to the electrode 31a. The electrode pin 36 a is electrically connected to one end of the modulation electrode 13 a via the electrode 31 a on the carrier 31. The electrode pin 36b is wire-bonded to the ground pattern of the carrier 31 and functions as a ground electrode. The electrode pin 36c and the electrode pin 36d are wire bonded to the thermistor 38 and the ground pattern of the carrier 31, respectively. The electrode pin 37a is wire bonded to the electrode 31c. The electrode pin 37 a is electrically connected to one end of the modulation electrode 13 b through the electrode 31 c on the carrier 31. The electrode pin 37b is wire-bonded to the ground pattern of the carrier 31 and functions as a ground electrode. The electrode pins 37c and 37d are wire-bonded to the Peltier element 32 to become a positive electrode and a negative electrode, respectively.

As shown in FIG. 6, an optical modulation device 40 is configured using the optical modulator module 30 configured as described above.
A DC power supply 41a and a high-frequency signal source 42 are connected to the electrode pins 36a and 36b. A DC power source 41b and a high-frequency signal source 42 are connected to the electrode pins 37a and 37b. A temperature controller 43 is connected to the electrode pins 36c, 36d, 37c, and 37d. A control circuit 44 for controlling the DC power sources 41a and 41b, the high-frequency signal source 42, and the temperature controller 43 is provided according to each condition of a drive condition table 45 described later, and the MZ type light modulation device 40 is configured. The drive condition table 45 is created at the manufacturing stage of the MZ type light modulation device and stored in a predetermined memory or the like.

The DC power supply 41 a applies the first DC bias V _DC1 described in the drive condition table 45 between the electrode pins 36 a and 36 b under the control of the control circuit 44. RF signal source 42 is controlled by the control circuit 44 applies a first modulated signal is a high frequency signal amplitude V _pp described in the driving condition table 45 electrode pins 36a, between 36b.
The DC power supply 41 b applies the second DC bias V _DC2 described in the drive condition table 45 between the electrode pins 37 a and 37 b under the control of the control circuit 44. RF signal source 42 is controlled by the control circuit 44 applies a second modulated signal is a high frequency signal amplitude V _pp described in the driving condition table 45 electrode pins 37a, between 37b.
By the DC power sources 41a and 41b and the high frequency signal source 42, the modulation electrode 13a has the first DC bias V _DC1 and the first modulation signal, and the modulation electrode 13b has the second DC bias V _DC2 and the second modulation signal. Are applied respectively. The first modulation signal and the second modulation signal are modulation signals having the same amplitude and opposite voltage changes.

  The temperature controller 43 measures the resistance of the thermistor 38 to observe the temperature of the modulator element 20, adjusts the current injected into the Peltier element 32 based on the observed temperature, and adjusts the temperature of the modulator element 20. adjust. In the temperature controller 43, the modulator element 20 adjusts the driving temperature T described in the driving condition table 45 under the control of the control circuit 44.

In the present embodiment, the first DC bias V _DC1 and the second DC bias V _DC2 are set to the same value by the control of the control circuit 44 based on the drive condition table 45. Therefore, as shown in FIG. 7, the DC power supply 41b may be omitted, and only the DC power supply 41a may be provided. In this case, the DC power supply 41a is connected to the electrode pins 36a and 36b and the electrode pins 37a and 37b. DC power supply 41a is controlled by the control circuit 44 applies a DC bias _{V DC} which is described in the drive condition table 45 electrode pins 36a, 36b and between the electrode pins 37a, between 37b. At this time, the DC bias _VDC having the same value is applied to the modulation electrodes 13a and 13b.

  Hereinafter, a method for manufacturing the light modulation device 40 will be described. FIG. 8 is a flowchart showing the method of manufacturing the MZ type light modulation device 40 in the order of steps.

First, a modulator element is manufactured (step 1). The process of Step 1 is as described above and is omitted here.
Next, the characteristics of the semiconductor Mach-Zehnder modulator for creating the drive condition table 45 are evaluated, and the drive condition table 45 is created (step S2).
In the driving condition table 45, the driving temperature T of the modulator element 20, the first DC bias V _DC1 , the second DC bias V _DC2 (in this embodiment, V _DC1 = V _DC2 ), and the amplitude V _{pp of the} high frequency signal are included. be written. V _DC1 , V _DC2 , and V _pp are values that can perform a favorable modulation operation in a state where the semiconductor Mach-Zehnder modulator 20 is adjusted to the driving temperature T.

Next, the optical modulator module 30 is produced (step S3).
As shown in FIG. 6, the semiconductor Mach-Zehnder modulator 20 is bonded onto the carrier 31, the carrier 31 is disposed on the Peltier element 32, and the Peltier element 32 is mounted in the module package 33.
The step S2 for creating the drive condition table 45 may be executed after the optical modulator module 30 is produced in step S3.

Thereafter, the MZ type light modulation device 40 is manufactured (step S4).
As shown in FIG. 7, DC power sources 41 a and 41 b, a high-frequency signal source 42, and a temperature controller 43 are connected to the optical modulator module 30, and a control circuit 44 is connected. The control circuit 44 is connected to a memory in which a driving condition table 45 is stored.

Hereinafter, step S2 for creating the drive condition table 45 will be described in detail.
Consider the case where the first and second modulation signals are applied to the modulation electrodes 13a and 13b of the arms 12a and 12b as in the present embodiment so that the voltage changes are opposite to each other. At this time, in order to perform a good modulation operation with a high extinction ratio, it is necessary to appropriately adjust the phase difference between the arms 1 and 2. That is, the first modulation signal applied to the modulation electrode 13a is on level (higher voltage level), and the second modulation signal applied to the modulation electrode 13b is off level (lower voltage level). In this case, the light signal level is set to the on level. Conversely, when the first modulation signal is off level and the second modulation signal is on level, the light signal level is set to off level.

  In the semiconductor Mach-Zehnder modulator as in the present embodiment, for example, when the light input port is the upper 11a and the output port is the lower 14a, the light that has passed through the arms 12a and 12b passes through the output port 14a. When the phase difference at the time of multiplexing is 0 (or 2Nπ, N is an integer), light is output from the lower output port 14a. Therefore, the light signal level is turned on. When the phase difference when the light that has passed through the arms 12a and 12b is combined at the output port 14a is π (or (2N + 1) π), the light is output from the upper output port and from the lower output port. Is hardly output. Therefore, the light signal level is turned off.

  Therefore, it is necessary to adjust the phase difference between the arms 12a and 12b so that the phase difference becomes 2Nπ when the first modulation signal is at the on level and the second modulation signal is at the off level. In other words, it can be said that the phase difference needs to be (2N + 1) π when the first modulation signal is off level and the second modulation signal is on level. In other words, it can be paraphrased that the phase difference needs to be approximately (2N + 0.5) π at the center bias that is the center value of the on level and the off level of the first and second modulation signals.

When the semiconductor Mach-Zehnder modulator 20 using the 2 × 2 MMI as an input / output coupler as in the present embodiment is ideally manufactured, the input port is set to the upper side 11a and the output port is set to the lower side. 14a, the phase difference of the light that has passed through the arms 12a and 12b is zero when no voltage is applied to the modulation electrodes 13a and 13b.
However, in reality, the phase difference is not exactly 0 but has a finite value Δθ due to a manufacturing error or the like. Since this Δθ is caused by a manufacturing error or the like, even if the semiconductor Mach-Zehnder modulator 20 having the same specification is manufactured, each semiconductor Mach-Zehnder modulator 20 has a different value. Therefore, in order to set the phase difference between the arms 12a and 12b to (2N + 0.5) π using the DC bias (center bias), the initial phase difference between the arms 12a and 12b due to this manufacturing error is taken into consideration. It needs to be put in. That is, the amount of phase difference between the arms 12a and 12b generated by applying a DC bias needs to be (2N + 0.5) π−Δθ.

FIG. 9 shows a rewritten version of FIG. 2 for explaining a driving method for adjusting the phase difference by increasing / decreasing the DC bias.
In the present embodiment, the DC bias is increased or decreased in a state where the DC bias is matched between the arms 12a and 12b. However, since the lengths of the modulation electrodes 13a and 13b are different, even if the same voltage is applied, the arms 12a and 12b The amount of phase change that occurs is different. As a result, the phase difference between the arms 12a and 12b also changes by increasing or decreasing the center bias value. As the DC bias of both arms increases, the phase difference between the arms 12a and 12b increases. Therefore, if a DC bias is applied to the modulation electrodes 13a and 13b until the phase difference reaches, for example, 0.5π−Δθ, the phase difference between the arms 12a and 12b can be appropriately adjusted. For example, if the initial phase difference is Δθ ′ in another modulator element 20, the DC bias of the arms 12a and 12b is set to V ′ in FIG. 9 so that the phase difference becomes a value of 0.5π−Δθ ′. Just do it.

  As described above, since the value of Δθ has a different value for each semiconductor Mach-Zehnder modulator 20, the required DC bias value differs for each modulator element 20. When the DC bias is matched between the modulation electrodes 13a and 13b as in the present embodiment, the ratio of the amount of phase change generated in the arms 12a and 12b can be fixed regardless of the DC bias value. For this reason, the characteristics of the wavelength chirp are hardly changed. As a result, it is possible to achieve both proper control of the phase difference between the arms 12a and 12b and precise control of the wavelength chirp, and good modulation characteristics can be obtained in the modulator element 20.

The adjustment of the phase difference by the DC bias according to the present embodiment can be considered as described above in principle. However, in the actual adjustment, it is difficult to directly observe the change in the phase of each arm. Therefore, in the present embodiment, in step S1, the phase difference is adjusted by the following procedure to create a DC bias driving condition table.
The first DC bias (or the second DC bias) is changed (swept) from 0 V to several V in a state where the first DC bias and the second DC bias applied to the modulation electrodes 13a and 13b are matched. . At this time, when the intensity of the light output from the output port (in this case, the output port 14a) is observed, for example, a so-called extinction curve as shown in FIG. 10 is obtained. This extinction curve represents a state in which the state of interference changes due to the phase difference between the arms and the output light intensity changes. The phase difference between the arms 12a and 12b is 2Nπ at the maximum portion and (( 2N + 1) π. The phase difference between the arms 12a and 12b is (2N + 0.5) π when the center bias V _cb that is the midpoint between the maximum and minimum is applied. Therefore, the above-described phase adjustment is performed by matching the center bias V _cb with the first DC bias applied to the arm 12a. In the present embodiment, since the first DC bias applied to the arm 12a and the second DC bias applied to the arm 12b have the same value, the second DC bias applied to the arm 12b is also set to the center bias V _cb. It ’s fine.

  If the semiconductor Mach-Zehnder modulator 20 like this embodiment is ideally manufactured, this extinction curve is a solid line in FIG. 10 when the initial phase difference between the arms 12a and 12b is zero. As shown, the extinction curve reaches a local maximum when the bias is zero. When there is a manufacturing error in the modulator element 20 and there is an initial phase difference generated thereby, the extinction curve shifts in the horizontal axis direction as shown by the broken line in FIG. To have.

Therefore, when the semiconductor Mach-Zehnder modulator 20 is ideally manufactured as indicated by the solid line, the DC bias applied to the arm 12a may be set to the value of the center bias V _cb in FIG. When there is an initial phase difference between the arms 12a and 12b as indicated by the broken line, the first DC bias V _DC1 applied to the arm 12a may be set to the value of the center bias V _cb ′ in FIG. As described above, since the center bias that is the middle point between the maximum and minimum is changed due to the initial phase difference generated depending on the manufacturing variation of the semiconductor Mach-Zehnder modulator 20, the DC bias whose phase is appropriately adjusted can be obtained. Different for each semiconductor Mach-Zehnder modulator 20.

The adjustment of the phase difference based on the evaluation of the extinction curve as described above is difficult to adjust while actually performing the modulation operation. Therefore, in the present embodiment, the extinction curve as shown in FIG. 10 is evaluated at the manufacturing stage of the semiconductor Mach-Zehnder modulator 20 (or the modulator element module 30), and the first and second DC biases V _DC1 , V _DC2 (V _DC1 = V _{DC2 in} this embodiment) is determined in advance under the drive temperature T. The determined V _DC1 and V _DC2 are written in the drive condition table 45 together with the drive temperature T.

  In general, in a semiconductor optical waveguide, the phase change of the waveguide when a voltage is applied hardly deteriorates with time. Therefore, if a drive condition table is created as described above at the manufacturing stage and the semiconductor Mach-Zehnder modulator 20 is driven based on the table, it is possible to maintain a good modulation operation semi-permanently.

  In general, it is desirable that the MZ type optical modulator be operable over the entire C-band band of the communication wavelength band, for example. The MZ type optical modulator has different characteristics for each wavelength due to the wavelength dependence of the amount of phase change when a voltage is applied to the semiconductor optical waveguide. As a result, the condition of the DC bias having the optimal phase as described above varies depending on the wavelength. Therefore, it is basically necessary to separately evaluate the DC bias condition for each wavelength. Since the in-phase DC bias has a relatively simple relationship with the wavelength, the extinction curve is evaluated at several wavelengths to obtain the optimum value of the DC bias, and the evaluated wavelength is based on the result. What is necessary is just to obtain | require DC bias complementing about the wavelength of between. For example, the DC bias may be adjusted by evaluating the extinction curve as described above at the three wavelengths of the short-wave side and the long-wave side end and center of the C-band. For the DC bias at the wavelength between the adjusted wavelengths, the condition of the DC bias at all wavelengths is compensated by using linear interpolation or complementing the relationship between the DC bias and the wavelength with a quadratic function. Just ask.

In step S1, the first and second extinction ratios can be obtained under the condition that the first and second DC biases V _DC1 and V _DC2 are set to the center bias V _bc obtained at each wavelength as described above. The amplitude (V _pp ) of the second modulation signal is determined in advance. Then, the determined Vpp is described in the drive condition table 45.
The amplitude of the first and second modulation signals may be adjusted by, for example, observing the eye pattern of the modulation waveform and determining V _pp that is large enough to ensure a sufficient extinction ratio. Regard V _pp, the optimum value differs for each wavelength in the semiconductor Mach-Zehnder modulator 20. In this case, as in the determination of the first and second DC biases V _DC1 and V _DC2 , the optimum V _pp is experimentally obtained at several wavelengths, and the wavelength between them is complemented by a simple relationship. .

Hereinafter, based on the driving condition table created as described above, the driving method of the light modulation device according to the present embodiment will be described. FIG. 11 is a flowchart showing a method of driving the light modulation device according to the first embodiment.
First, the control circuit 44 reads the driving temperature T of the semiconductor Mach-Zehnder modulator 20 from the driving condition table 45 (step S11).
Next, the temperature controller 43 adjusts the temperature of the semiconductor Mach-Zehnder modulator 20 to a constant driving temperature T (for example, 25 ° C.) under the control of the control circuit 44 (step S12).

Next, the control circuit 44 reads the first and second DC biases V _DC1 and V _DC2 to be applied to the arms 12a and 12b from the drive condition table 45 (step S13).
Next, under the control of the control circuit 44, the DC power sources 41a and 41b read the first and first read into the modulation electrodes 13a and 13b of the arms 12a and 12b via the electrode pins 36a and 36b and the electrode pins 37a and 37b. 2 DC biases V _DC1 and V _DC2 are applied (step S14).

Next, the control circuit 44 reads the amplitude V _{pp of the} high frequency signal from the drive condition table 45 (step S15).
Next, the high-frequency signal source 42 is controlled by the control circuit 44 with the amplitude V _pp read into the modulation electrodes 13a and 13b of the arms 12a and 12b via the electrode pins 36a and 36b and the electrode pins 37a and 37b. First and second modulation signals, which are high-frequency signals whose voltage changes are opposite to each other, are applied (step S16).
With the above procedure, the modulator element 20 can perform a good modulation operation.

In this embodiment, the case where 2 × 2 MMI couplers are used as the input and output optical couplers has been described as the structure of the semiconductor Mach-Zehnder modulator 20, but the structure is not limited to this.
The input-side optical coupler may be any one that can split the input light into two, and the output-side optical coupler may be any one that couples the light incident from the two arms. For example, the same effect can be obtained with an optical coupler using a 1 × 2 MMI coupler, a Y-branch coupler, or a directional coupler. However, the ideal phase difference that occurs when no DC bias is applied differs depending on the combination of optical couplers used, and the DC bias needs to be adjusted accordingly. For example, when a 1 × 2 MMI coupler is used on the input side and a 2 × 2 MMI coupler is used on the output side, the phase difference between the arms when no voltage is applied to the modulation electrode is ideally 0.5π. It becomes. Therefore, it may be considered that the necessary phase adjustment amount is only the initial phase difference generated due to a manufacturing error or the like.

  As described above, according to the present embodiment, it is possible to realize optimal control of the phase difference between the arms 12a and 12b and precise control of the wavelength chirp characteristics with a simple element configuration. Light modulation with a small size and good characteristics is realized.

(Second Embodiment)
Although the present embodiment is directed to an optical modulation device using a semiconductor Mach-Zehnder modulator, as in the first embodiment, it is different in that the modulation mode is different.
In the light modulation device according to the present embodiment, the configurations of the semiconductor Mach-Zehnder modulator, the light modulator module, and the light modulation device are substantially the same as those of the first embodiment. Therefore, also in this embodiment, it demonstrates using the drawing and code | symbol of 1st Embodiment suitably.

In order to perform the modulation operation by changing the phase of the light passing through the arms 12a and 12b, the arm 12a is supplied with the first DC bias and the first modulation signal from the modulation electrode 13a, and the arm 12b is supplied with the modulation electrode 13b. A second DC bias and a second modulation signal are applied, respectively.
In the arms 12a and 12b, the ratio of the product of the length of the modulation electrode 13a and the first DC bias and the product of the length of the modulation electrode 13b and the second DC bias is maintained at a constant value. A first DC bias and a second DC bias are applied.

In the present embodiment, the first DC bias V _DC1 applied to the modulation electrode 13a and the second DC bias V _DC2 applied to the modulation electrode 13b are fixed at V _DC1 : V _DC2 = L ₂ : L ₁ . Under this condition, a first modulation signal and a second modulation signal having the same amplitude and opposite directions of voltage change are applied to the modulation electrodes 13a and 13b. As a result, during the modulation operation, the arms 12a and 12b change in phase by approximately the same amount in opposite directions, and a satisfactory zero chirp operation can be realized.

The details are as follows. The modulation electrode 13a is formed to have a length L ₁ = 1.5 mm, and the modulation electrode 13b is formed to have a length L ₂ = 0.5 mm. V _DC1 : V _DC2 is fixed at 1: 3. Accordingly, the ratio η ₁ : η ₂ of the phase change efficiency in the arms 12a and 12b is fixed to the following value.
η ₁ : η ₂ = L ₁ V _DC1 : L ₂ V _DC2 = 1.5 × 1: 0.5 × 3 = 1: 1
That is, the phase change efficiencies in the arms 12a and 12b have the same value.
As in the first embodiment, the first and second DC biases are increased or decreased in the arms 12a and 12b while the relationship of the first DC bias V _DC1 : second DC bias V _DC2 is fixed to 1: 3. By doing so, the phase difference between the arms 12a and 12b can be changed and adjusted.
As in the first embodiment, when the first and second DC biases are increased, the phase difference increases, so that the phase difference between the arms 12a and 12b can be optimally adjusted.

In the present embodiment, when the drive condition table is created in step S2 of FIG. 8 of the first embodiment, the phase difference is adjusted by the following procedure.
The first DC bias is changed from 0 V to several V with the relationship between the first and second DC biases V _DC1 and V _DC2 applied to the modulation electrodes 13a and 13b fixed at 1: 3. At this time, when the intensity of light output from the output port (in this case, the output port 14a) is observed, an extinction curve as shown in FIG. 12, for example, is obtained. Similar to the first embodiment, the above-described phase adjustment is performed by matching the center bias V _cb of FIG. 12 with the first DC bias applied to the arm 12a. In this embodiment, since the relationship between the first and second DC biases V _DC1 and V _DC2 applied to the modulation electrodes 13a and 13b is fixed at 1: 3, the second DC bias applied to the arm 12b is A value of V _cb × 3 may be used.

  If the modulator element 20 is ideally manufactured, this extinction curve is zero when the initial phase difference between the arms 12a and 12b is zero as shown by the solid line in FIG. By the way, it becomes maximum value. If there is a manufacturing error in the modulator element 20 and there is an initial phase difference generated thereby, the extinction curve shifts in the horizontal axis direction as shown by a broken line in FIG. It becomes like this.

Therefore, when the modulator element 20 is ideally manufactured as indicated by a solid line, the first DC bias applied to the arm 12a may be set to the value of the center bias V _cb in FIG. When an initial phase difference exists between the arms 12a and 12b as indicated by a broken line, the first DC bias V _DC1 applied to the arm 12a may be set to the value of the center bias V _cb ′ in FIG. Thus, since the center bias at the midpoint between the maximum and minimum is changed by the initial phase difference generated depending on the manufacturing variation of the modulator element 20, the DC bias whose phase is appropriately adjusted is also applied to the modulator element. Different for every 20.

The first and second DC biases V _DC1 and V _DC2 (V _DC2 = 3V _{DC1 in} this embodiment) determined as described above are driven together with the amplitude V _pp and the driving temperature T of the first and second modulation signals. Enter in the condition table.
The method of driving the light modulation device according to the present embodiment is the same as that in FIG. 11 of the first embodiment. That is, in steps S11 to S14, the driving temperature T is read from the driving condition table to adjust the temperature, V _DC1 and V _DC2 are read and applied to the modulation electrodes 13a and 13b, the amplitude V _pp is read and the voltage changes mutually. Apply a high-frequency signal that is reversed.

  As described above, according to the present embodiment, it is possible to realize optimal control of the phase difference between the arms 12a and 12b and precise control of the wavelength chirp characteristics with a simple element configuration. Light modulation with a small size and good characteristics is realized.

(Third embodiment)
Although the present embodiment is directed to an optical modulation device using a semiconductor Mach-Zehnder modulator, as in the first embodiment, it is different in that the modulation mode is different.
In the light modulation device according to the present embodiment, the configurations of the semiconductor Mach-Zehnder modulator, the light modulator module, and the light modulation device are substantially the same as those of the first embodiment. Therefore, also in this embodiment, it demonstrates using the drawing and code | symbol of 1st Embodiment suitably.

In order to perform the modulation operation by changing the phase of the light passing through the arms 12a and 12b, the arm 12a is supplied with the first DC bias and the first modulation signal from the modulation electrode 13a, and the arm 12b is supplied with the modulation electrode 13b. A second DC bias and a second modulation signal are applied, respectively.
In the arms 12a and 12b, the ratio of the product of the length of the modulation electrode 13a and the first DC bias and the product of the length of the modulation electrode 13b and the second DC bias is maintained at a constant value. A first DC bias and a second DC bias are applied. In the present embodiment, in order to realize a desired negative char program operation, the above-described ratio in the first and second modulation signals is maintained at 5: 1, for example.

In the present embodiment, the ratio of the length of the modulation electrodes 13a _{L 1} and the length _{L 2} of the modulation electrode 13b is 2: to be 1, to form the modulation electrode 13a, a 13b. Specifically, the modulation electrode 13a is formed to have a length L ₁ = 1.5 mm, and the modulation electrode 13b is formed to have a length L ₂ = 0.75 mm. The first DC bias V _DC1 applied to the modulation electrode 13a and the second DC bias V _DC2 applied to the modulation electrode 13b are fixed at V _DC1 : V _DC2 = 2.5: 1. Under this condition, a first modulation signal and a second modulation signal having the same amplitude and opposite directions of voltage change are applied to the modulation electrodes 13a and 13b.

The ratio η ₁ : η ₂ of the phase change efficiency in the arms 12a and 12b is fixed to the following value.
η ₁ : η ₂ = L ₁ V _DC1 : L ₂ V _DC2 = 1.5 × 2.5: 0.75 × 1 = 5: 1
As a result, a 5: 1 phase change occurs in the directions opposite to each other in the arms 12a and 12b, and a favorable negative chirp operation can be realized.

As shown in FIG. 13, the first and second DC biases are increased or decreased in the arms 12a and 12b while the relationship of the first DC bias V _DC1 : second DC bias V _DC2 is fixed at 2.5: 1. By doing so, the phase difference between the arms 12a and 12b can be changed and adjusted. In FIG. 13, when the first and second DC biases are V _DC1 and V _DC2 , the phase difference between the arms 12 a and 12 b is Δφ, and when the first and second DC biases are V _DC1 ′ and V _DC2 ′, the phase difference is Δφ ′. .
As in the first embodiment, when the first and second DC biases are increased, the phase difference increases, so that the phase difference between the arms 12a and 12b can be optimally adjusted.

In the present embodiment, when the drive condition table is created in step S2 of FIG. 8 of the first embodiment, the phase difference is adjusted by the following procedure.
With the relationship between the first and second DC biases V _DC1 and V _DC2 applied to the modulation electrodes 13a and 13b fixed at 2.5: 1, the second DC bias is changed from 0V to several volts. At this time, when the intensity of the light output from the output port (in this case, the output port 14a) is observed, for example, an extinction curve as shown in FIG. 14 is obtained. Similarly to the first embodiment, the above-described phase adjustment is performed by matching the center bias V _cb of FIG. 14 with the second DC bias applied to the arm 12b. In the present embodiment, since the relationship between the first and second DC biases V _DC1 and V _DC2 applied to the modulation electrodes 13a and 13b is fixed at 2.5: 1, the first DC applied to the arm 12a. The bias may be a value of V _cb × 2.5.

  If the modulator element 20 is ideally manufactured, the extinction curve is zero when the initial phase difference between the arms 12a and 12b is zero as shown by the solid line in FIG. By the way, it becomes maximum value. When there is a manufacturing error in the modulator element 20 and there is an initial phase difference caused by the manufacturing error, the extinction curve shifts in the horizontal axis direction as shown by the broken line in FIG. It becomes like this.

Therefore, when the modulator element 20 is ideally manufactured as indicated by the solid line, the second DC bias applied to the arm 12b may be set to the value of the center bias V _cb in FIG. When an initial phase difference exists between the arms 12a and 12b as indicated by the broken line, the second DC bias V _DC1 applied to the arm 12b may be set to the value of the center bias V _cb ′ in FIG. Thus, since the center bias at the midpoint between the maximum and minimum is changed by the initial phase difference generated depending on the manufacturing variation of the modulator element 20, the DC bias whose phase is appropriately adjusted is also applied to the modulator element. Different for every 20.

The first and second DC bias _{V DC1, V DC2} determined _(V DC1 = _{2.5V DC2} in this embodiment) As described above, the first and second modulation signal amplitude _{V pp} and drive temperature T In addition, it is described in the driving condition table.
The driving method of the MZ type light modulation device according to the present embodiment is the same as that in FIG. 11 of the first embodiment. That is, in steps S11 to S14, the driving temperature T is read from the driving condition table to adjust the temperature, V _DC1 and V _DC2 are read and applied to the modulation electrodes 13a and 13b, the amplitude V _pp is read and the voltage changes mutually. Apply a high-frequency signal that is reversed.

  As described above, according to the present embodiment, it is possible to realize optimal control of the phase difference between the arms 12a and 12b and precise control of the wavelength chirp characteristics with a simple element configuration. Light modulation with a small size and good characteristics is realized.

(Fourth embodiment)
FIG. 15 is a schematic plan view showing a semiconductor Mach-Zehnder modulator used in the light modulation device according to the fourth embodiment.
FIG. 16 is a schematic plan view showing a modulator module on which the semiconductor Mach-Zehnder modulator of FIG. 15 is mounted.
FIG. 17 is a schematic plan view showing an optical modulation device on which the modulator module of FIG. 16 is mounted.

  As shown in FIG. 15, the semiconductor Mach-Zehnder modulator 50 used in the optical modulation device (optical transmitter) according to the present embodiment includes the first optical coupler 11, the arms 12a and 12b, and the second optical coupler 14. It comprises a semiconductor optical waveguide and modulation electrodes 13a and 13b. In the present embodiment, the first optical coupler 11, the arms 12a and 12b, and the second optical coupler 14 are the same as those of the modulator element 20 of the first embodiment. The cross-sectional configurations of the arms 12a and 12b are also the same as in FIG. 4B of the first embodiment.

In the present embodiment, the modulation electrodes 13a and 13b are formed to have the same length. Further, the additional electrode 51 is formed only on the arm 12b. The additional electrode 51 is formed by patterning simultaneously with the modulation electrodes 13a and 13b.
The first modulation signal and the second modulation signal from the modulation electrodes 13a and 13b are applied to the arms 12a and 12b. At this time, the product of the length L ₁ of the modulation electrode 13a and the first DC bias V _DC1 and the length (L ₂ + L) of the length L ₂ of the modulation electrode 13b plus the length L ₃ of the additional electrode 51 ₃ ) The ratio of the product of the second DC bias V _DC2 is kept constant. Here, “the length of the additional electrode” is the same as the “length of the modulation electrode” in the longitudinal direction of the arm of the portion electrically connected to the waveguide constituting the arm of the additional electrode. Say the length along.

As shown in FIG. 15, an optical modulator module 60 is configured using the semiconductor Mach-Zehnder modulator 50 configured as described above.
The optical modulator module 60 is configured in substantially the same manner as the optical modulator module 30 of FIG. 5 of the first embodiment except for the semiconductor Mach-Zehnder modulator 50, but an electrode 31e and an inductor 52 are attached to the carrier 31. Is different.

  The electrode 31e is wire bonded to one end of the additional electrode 51. The inductor 52 is, for example, about 100 μH, and is provided so as to electrically connect the electrode 31d and the electrode 31e. The module package 33 is not provided with an electrode pin for applying a voltage independently to the additional electrode 51. Therefore, in the present embodiment, forming the additional electrode 51 does not increase the number of electrode pins.

As shown in FIG. 17, an optical modulator 70 is configured using the optical modulator module 60 configured as described above.
The light modulation device 70 is configured in substantially the same manner as the light modulation device 40 of FIG. 6 of the first embodiment except for the light modulator module 60.

  In the optical modulation device 70, the DC power sources 41a and 41b and the high-frequency signal source 42 cause the modulation electrode 13a to have the first DC bias and the first modulation signal, and the modulation electrode 13b to have the second DC bias and the second DC signal. Each modulation signal is applied. At this time, in the arm 12b in which the modulation electrode 13b and the additional electrode 51 are provided, the second DC bias is applied to both the modulation electrode 13b and the additional electrode 51. On the other hand, the second modulation signal, which is a high-frequency signal, is hardly applied to the additional electrode 51 by the inductor 52.

  This is equivalent to the case where the modulation signals are effectively applied to the modulation electrodes having the same electrode length by the arms 12a and 12b. On the other hand, when the DC bias is viewed effectively, it is equivalent to applying each DC bias to electrodes having different electrode lengths by the arms 12a and 12b.

In the present embodiment, the first and second DC biases applied to the modulation electrodes 13a and 13b are matched by the arms 12a and 12b, and the first and second modulation signals applied to the modulation electrodes 13a and 13b are The amplitudes are equal and the direction of voltage change is reversed. In this case, since the lengths of the modulation electrodes 13a and 13b to which each modulation signal is effectively applied are equal, the efficiency of phase change with respect to the first and second modulation signals is equal between the arms 12a and 12b. As a result, a zero chirp operation in which the amount of phase change is equal between the arms 12a and 12b is realized.
Further, regarding the phase difference between the arms due to the application of the first and second DC biases that determine the phase difference between the arms 12a and 12b, the first and second biases are applied to the electrodes having different effective lengths. Will do. Therefore, as in the first embodiment, the phase difference between the arms 12a and 12b can be adjusted by increasing or decreasing the first and second DC biases while keeping them equal to each other.

  In the present embodiment, the phase difference between the arms 12a and 12b can be optimally adjusted while maintaining the zero chirp operation. Further, since the amplitudes of the first and second modulation signals to be applied are the same in the arms 12a and 12b, and the magnitudes of the first and second DC bias to be applied are the same in the arms 12a and 12b, it is simple. Control can be performed.

In the present embodiment, as in the first embodiment, the first DC bias V _DC1 and the second DC bias V _DC2 are set to the same value by the control of the control circuit 44 based on the drive condition table 45. Therefore, as shown in FIG. 18, the DC power supply 41b may be omitted, and only the DC power supply 41a may be provided. In this case, a DC power supply 41a is connected between the electrode pins 36a and 36b and between the electrode pins 37a and 37b. DC power supply 41a is controlled by the control circuit 44 applies a DC bias _{V DC} which is described in the drive condition table 45 electrode pins 36a, 36b and between the electrode pins 37a, between 37b. At this time, the same value of DC bias _VDC is applied to the modulation electrode 13a, the modulation electrode 13a, and the additional electrode 51.

  As described above, according to the present embodiment, it is possible to realize optimal control of the phase difference between the arms 12a and 12b and precise control of the wavelength chirp characteristics with a simple element configuration. Light modulation with a small size and good characteristics is realized.

In the first to fourth embodiments, when the ratio between the first DC bias V _DC1 and the second DC bias V _DC2 is a constant value, V _DC1 and V _DC2 that are single constant values are used. It has been described in the drive condition table, and the mode in which the light modulation device is driven based on this is shown.
Other aspects can be considered in this regard. For example, the drive condition table 1 according to the first embodiment: 1 and _{V DC1, V DC2} made, 1 according to the second embodiment: 3 and _{V DC1, V DC2} made, 2 according to the third embodiment. V _DC1 and V _DC2 which are 5: 1 are described. Then, depending on the use state of the MZ type light modulation device, appropriate V _DC1 and V _DC2 may be read from the drive condition table to obtain a desired 0 chirp operation or negative chirp operation.

  According to this case, it is possible to realize optimal control of the phase difference between the first and second optical waveguides and precise control of the wavelength chirp characteristic with a simple element configuration, and the device size is small. In addition, light modulation with good characteristics is realized.

Claims (19)

A light modulation device using a semiconductor Mach-Zehnder type modulator for branching and propagating input light, and combining and outputting the propagated light,
The semiconductor Mach-Zehnder modulator is
A first optical waveguide and a second optical waveguide that propagate the branched light;
A first electrode for applying a first modulation signal and a first DC bias to the first optical waveguide;
A second electrode for applying a second modulation signal and a second DC bias to the second optical waveguide;
The first electrode is configured to maintain a constant ratio between a product of the length of the first electrode and the first DC bias and a product of the length of the second electrode and the second DC bias. And a second DC bias are applied.   The length of the first electrode is different from the length of the second electrode, the product of the length of the first electrode and the first DC bias, and the length of the second electrode 2. The light modulation device according to claim 1, wherein a ratio of the product with the second DC bias is 1: 1.   The ratio of the product of the length of the first electrode and the first DC bias to the product of the length of the second electrode and the second DC bias is 1: 4 to 1: 5. The light modulation device according to claim 1. The semiconductor Mach-Zehnder modulator further includes a third electrode formed separately from the second electrode for applying a voltage to the second optical waveguide,
The lengths of the first electrode and the second electrode are the same,
The second electrode and the third electrode are connected via an inductor;
The ratio of the product of the length of the first electrode and the first DC bias to the product of the length of the second electrode and the second DC bias is 1: 1. The light modulation device according to claim 1.   2. The optical modulation device according to claim 1, wherein the first modulation signal and the second modulation signal are alternating-current signals having the same amplitude and opposite directions of voltage change. A memory storing a table describing the first DC bias and the second DC bias defined in advance so as to keep the ratio at a constant value;
A first power source connected to the first electrode;
A second power source connected to the second electrode;
The first power source and the second power source apply the first DC bias and the second DC bias described in the table to the first electrode and the second electrode. The light modulation device according to claim 1, wherein the light modulation device is characterized.   Controlling the first power source and the second power source so that the first DC bias and the second DC bias described in the table are applied to the first electrode and the second electrode. The light modulation device according to claim 6, further comprising a control unit that performs the control. A method of driving a light modulation device using a semiconductor Mach-Zehnder type modulator that branches and propagates input light, combines and outputs the propagated light,
A first optical waveguide and a second optical waveguide that propagate the branched light;
A first electrode for applying a first modulation signal and a first DC bias to the first optical waveguide;
A light modulation device comprising: a second electrode for applying a second modulation signal and a second DC bias to the second optical waveguide;
The first electrode is configured to maintain a constant ratio between a product of the length of the first electrode and the first DC bias and a product of the length of the second electrode and the second DC bias. A driving method of an optical modulation device, wherein the first DC bias and the second DC bias are applied to the optical waveguide and the second optical waveguide.   The ratio of the product of the length of the first electrode and the first DC bias to the product of the length of the second electrode and the second DC bias is 1: 1. A method for driving the light modulation device according to claim 8. The ratio of the product of the length of the first electrode and the first DC bias to the product of the length of the second electrode and the second DC bias is 1: 4 to 1: 5. The method of driving the light modulation device according to claim 8. The semiconductor Mach-Zehnder modulator further includes a third electrode formed separately from the second electrode,
The lengths of the first electrode and the second electrode are formed to be the same,
The second electrode and the third electrode are connected via an inductor;
The ratio of the product of the length of the first electrode and the first DC bias to the product of the length of the second electrode and the second DC bias is 1: 1. A method for driving the light modulation device according to claim 8.   Applying the first modulation signal and the second modulation signal, which are alternating current signals having the same amplitude and opposite directions of voltage change, to the first optical waveguide and the second optical waveguide, A method for driving the light modulation device according to claim 8. Using a table describing the first DC bias and the second DC bias defined in advance so as to keep the ratio constant.
The light modulation device comprises:
A first power source connected to the first electrode;
A second power source connected to the second electrode;
Applying the first DC bias and the second DC bias described in the table to the first electrode and the second electrode by the first power source and the second power source. The driving method of the light modulation device according to claim 8, wherein the driving method is a feature. The light modulation device further includes a control unit that controls the first power source and the second power source,
The control unit applies the first DC bias and the second DC bias described in the table to the first electrode and the second electrode, so that the first power source and the second DC bias are applied to the first electrode and the second electrode. 14. The method of driving a light modulation device according to claim 13, wherein the power supply of the second power supply is controlled. A method of manufacturing a light modulation device using a semiconductor Mach-Zehnder type modulator for branching and propagating input light, and combining and outputting the propagated light,
When producing the semiconductor Mach-Zehnder modulator,
Forming a first optical waveguide and a second optical waveguide for propagating the branched light;
A first electrode for applying a first modulation signal and a first DC bias to the first optical waveguide on the first optical waveguide, and the second electrode on the second optical waveguide. Forming a second modulation signal and a second electrode for applying a second DC bias to the optical waveguide, and further comprising:
The first DC having a constant ratio between the product of the length of the first electrode and the first DC bias and the product of the length of the second electrode and the second DC bias. A method of manufacturing a light modulation device, comprising: determining a bias and the second DC bias.   The semiconductor Mach-Zehnder type modulator is an electrode for applying a voltage to the second optical waveguide, is formed separately from the second electrode, and is connected to the second electrode via an inductor. The method of manufacturing a light modulation device according to claim 15, further comprising a third electrode formed.   16. The method of manufacturing an optical modulation device according to claim 15, further comprising a step of creating a table that describes the determined first DC bias and the second DC bias.   The table includes the determined first DC bias and the second DC bias in addition to the determined first DC bias and the second DC bias, and the determined first DC bias and the second DC bias. The method for manufacturing an optical modulation device according to claim 17, wherein a driving temperature at the time of applying to the second optical waveguide is described. Determining the first DC bias and the second DC bias;
Light output when the first DC bias and the second DC bias applied to the first optical waveguide and the second optical waveguide are changed while the ratio is maintained at a constant value. 16. The light modulation according to claim 15, wherein a median value of the maximum value and the minimum value of the extinction curve is determined as the first DC bias or the second DC bias using the extinction curve of Device manufacturing method.

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